Sulfur dioxide (SO.sub.2) is found in gases vented from industrial plants involved in roasting, smelting and sintering sulfide ores, such as chalcopyrite (CuFeS.sub.2), iron pyrites (FeS.sub.2) or pyrrhotite (Fe.sub.7 S.sub.8), or gases from power plants burning sulfur-bearing coal or in exit gases from other industrial operations involved in the combustion of sulfur-bearing fuels, such as fuel oil. Air pollution as the result of SO.sub.2 emissions from these gases represents not only a health hazard by contaminating the surrounding atmosphere, but results in a loss of valuable sulfur values. Therefore, it is desirable to recover SO.sub.2 from such gases preferably in the form of elemental sulfur in order to control SO.sub.2 emissions economically.
The reduction of sulfur dioxide to elemental sulfur has been investigated extensively over a period of many years and there have been published a myriad of references on this subject. For instance, typical of such disclosures include U.S. Pat. Nos. 3,199,955; 3,927,191; 3,928,547 and 4,039,650 and the references disclosed therein and cited during the prosecution of the patent applications which matured into these patents. U.S. Pat. No. 3,199,955 discloses a variety of processing and equipment schemes for the reduction of SO.sub.2 with a hydrocarbon reductant. This patent teaches that the reactants should be preheated to a temperature sufficient to initiate and maintain the reduction reaction. U.S. Pat. No. 3,927,191 discloses a three stage reaction system wherein the sulfur dioxide is introduced into the system in two of the three reactor stages in certain ratios to reduce the heat of reaction and permit higher gas space velocities. U.S. Pat. No. 3,928,547 discloses the use of elemental sulfur vapors during the initiation of the reduction of gaseous sulfur dioxide with a gaseous hydrocarbon reducing agent at elevated temperatures. In U.S. Pat. No. 4,039,650 there is disclosed another process wherein a staged reactor system is employed for the reduction of sulfur dioxide to elemental sulfur. In U.S. Pat. Nos. 3,928,547 and 4,039,650 it is disclosed that the presence of elemental sulfur vapors during the initiation of the reduction reaction permits the use of lower temperatures to initiate the reduction of sulfur dioxide with a hydrocarbon reducing agent, such as methane, than previously disclosed. Also, the presence of this elemental sulfur vapor has a moderating effect on the temperature rise during the reduction reaction.
As a result of the shortage of methane, particularly natural gas which contains a high proportion of methane, alternate reductants have been investigated for use in commercial installations for the reduction of sulfur dioxide to sulfur. However, when higher hydrocarbons were employed as the reductant, it has been observed that carbonaceous deposits form on the catalyst surfaces and that it is necessary to operate at higher operating temperatures than usual to avoid carbon deposition within the reactor. Such higher temperatures adversely affect the catalyst life. For instance, when using the same reactor system as in U.S. Pat. Nos. 3,928,547 and 4,039,650, it has been found that when butane is used as the hydrocarbon reductant, initiation temperatures of at least 1700.degree. F. are necessary to obviate carbon deposits on the catalyst surfaces.
It has previously been reported, U.S. Pat. No. 3,892,677, that in a process for regenerating sulfur oxide acceptors, such as copper or copper oxide; wherein the material has been converted to copper sulfate during the acceptance reaction, the the sulfate may be subsequently decomposed by reacting it with a reducing gas, such as a hydrocarbon reducing gas, to regenerate the acceptor and obtain a gas considerably richer in sulfur dioxide than the flue gas from which it was removed. During this regeneration with a hydrocarbon reducing agent, it is reported that combustible material is deposited on the acceptor and that the higher the molecular weight of the hydrocarbon regenerating gas the greater the amount of combustible material deposited on the acceptor surfaces. Relatively large quantities of combustible material are reported to be deposited when saturated hydrocarbons of more than 4 carbon atoms per molecule are used as the regenerating gas and by burning these large quantities of combustible material at a high temperature the acceptor's life is reduced. The patentee reports obviating or reducing the quantity of combustible material deposited during regeneration by treating the acceptor with a reducing gas comprising a mixture of a hydrocarbon and an inert diluent, such as steam, carbon dioxide and nitrogen.
Also, natural gas which is a mixture comprising various concentrations of methane, ethane, propane, the butanes, the pentanes, the hexanes, the heptanes, the octanes, nitrogen and carbon dioxide, has been found to form carbonaceous deposits on catalyst surfaces when it contains in excess of certain concentrations of the higher molecular weight hydrocarbons. For instance, a natural gas composition which contains more than about 10 mol percent propane, or more than about 7 mol percent butane, or more than about 0.25 mol percent pentane, hexane, heptane or octane has been found to form carbonaceous deposits on the catalyst surface during its use as a reductant in the reduction of sulfur dioxide.
The reduction of sulfur dioxide using a hydrocarbon reducing agent initiated at the elevated temperatures used in the prior art requires control of the maximum temperature reached during the reduction reaction and the use of special catalytic materials that can withstand prolonged exposure to these high temperatures without adversely affecting the activity of the catalyst. The use of these high reaction temperatures also requires consideration of the type of equipment and the materials of construction which may be employed since the sulfur dioxide reduction reaction is highly exothermic--corresponding to an adiabatic temperature rise in the reactor of from about 200.degree. to about 750.degree. F. depending on the strength of the SO.sub.2 contained in the feed gas. Prior art process designs capitalize on the use of heat exchanging of a product gas with the feed gas to attain the required reaction initiation temperatures. However, this is not feasible because of materials of construction problems in the heat exchanger. In standard heat exchangers, the practical upper limit of heat exchanger temperature for product gases containing sulfur or other sulfur compounds in addition to SO.sub.2 is generally in the range of about 1000.degree. to 1200.degree. F. Because of the higher temperatures needed to obviate the carbon deposition problem when certain higher hydrocarbons are used or are present in reducing agents, specially designed equipment would have to be employed to provide the supplemental heat necessary for a satisfactory reaction.
The gas flow configuration or method of introducing the feed gases in the reactor system used in U.S. Pat. Nos. 3,199,955; 3,927,191; 3,928,546 and 4,039,650 disclose the preheating of the SO.sub.2 gas and gaseous reductant. In the first two mentioned patents the preheating is accomplished using varying types of heat exchange equipment, whereas in the latter two patents preheating is effected by subjecting the mixture of SO.sub.2 and reductant to an up-flow regenerator or first reactor and subsequently reacting the mixture in the same vessel to effect reduction of a portion of the SO.sub.2 to elemental sulfur and hydrogen sulfide, mixing this product gas stream with bypassed portions of SO.sub.2 gas and reductant and then splitting the resulting gas mixture into two gas streams which are passed through second and third reactors operated in parallel to effect further reduction of the SO.sub.2 to sulfur and hydrogen sulfide. Periodically, the flow in the first and third reactors is reversed to thereby subject these reactors to alternating heat absorbing and desorbing cycles, while flow in the second reactor is maintained in the same direction. The bypassed portions of SO.sub.2 gas and reductant around the first reactor maintain the inlet gas temperatures to the second and third reactors within desired ranges. This type of reactor system flow configuration is called a "three vessel, reactor-regenerator system." In U.S. Pat. No. 3,199,955 processing schemes different from the three vessel, reactor-regenerator system are suggested.
When the hydrocarbon reducing agent is a higher hydrocarbon containing three or more carbon atoms or a natural gas containing concentrations of higher hydrocarbons, as disclosed hereinabove, the use of these processing systems presents some problems in the reduction of SO.sub.2. One problem which has been observed is that carbonaceous deposits form on the surfaces of the catalyst used in the process. These deposits are believed to result from the cracking of the higher hydrocarbons in the reductant. In co-pending application Ser. No. 838,699, filed Oct. 3, 1977 a method is disclosed wherein carbon deposits on the catalyst surfaces are obviated by reacting the sulfur dioxide-containing gas and the hydrocarbon reducing agent at elevated temperatures in the presence of elemental sulfur vapors.